(1) Field of the Invention
The present invention relates generally to a process for the countercurrent, reactive distillation, esterification of organic acids or esters with lower alcohols (C1 to C8). Specifically, the present invention relates to lactic acid esterification using reactive distillation with the alcohols, particularly ethanol. The present invention also preferably provides for recycling of dimers and trimers and other short-chain oligomers of lactic acid to improve yields.
(2) Description of the Related Art
Reactive distillation was first described in U.S. Pat. No. 1,400,849 to Backhaus. Backhaus discloses a continuous process of manufacturing organic acid esters comprising passing an organic acid in countercurrent fashion to a gradually increasing concentration of an alcohol. Backhaus specifically teaches a process of manufacturing methyl acetate using reactive distillation where methyl acetate and methanol are collected from the top of the reactive distillation column. The methyl acetate and methanol are then separated by a device such as a dephlegmator and/or a second column. The catalyst is an acid which is consumed in the reaction. Other prior approaches to producing organic acid esters (McKetta, John J., et al., Energy, Costing Thermal Electric Power Plants to Ethanol, Encyclopedia of Chemical Processing and Design, Marcell Dekker, Inc.), Vol. 19 381-402 (1976); Kirk and Othmer, “Elastomers, Polyisoprene to Expert Systems”. Encyclopedia of Chemical Technology, 4th Edition, John Wiley & Sons, New York 9 755-780 (1997)) involve batch processes, which are labor intensive, can create waste, such as with the use of a mineral acid catalyst which is consumed, and require an excess quantity of alcohol (Kaimal, T. N. B., et al., U.S. Pat. No. 6,342,626).
Conventional continuous processes for making organic acid esters typically require a large amount of equipment and high energy costs. McKetta et al. (Encyclopedia of Chemical Processing and Design, Chapter 19, 381-402) disclose batch and continuous processes for the production of either volatile or non-volatile esters using acid resin catalysts. Kirk-Othmer (ibid) discloses batch and continuous processes for the production of esters using resin catalysts. Kirk-Othmer and McKetta et al. do not disclose a continuous process of producing lactic acid esters within a single reactive distillation column. U.S. Pat. No. 6,342,626 to Kaimal et al. discloses a catalyst-free process for the conversion of lactic acid to an alkyl ester which comprises simultaneous hydrolysis of dimers or higher polymers of lactic acid to free lactic acid and the esterification of the free lactic acid at a temperature in the range of 130° C. to 250° C. for four to eleven hours at a pressure of 5 to 25 kg/cm2.
Organic acid esters can also be made directly from polymer precursors; in particular, ethyl lactate can be made by reacting ethanol with the precursor dilactide. The dilactide is quite expensive, however, as it requires several reaction steps to produce and purify.
Batch reactive distillation has been used (Seo, Y., W., et al., Korean Journal of Chemical Engineering 16(5), 556-561 (1999); and Choi, J. I., et al., Journal of Chemical Engineering of Japan 32(2), 184-189 (1999)) for making esters. Seo et al. (Korean J. Chem. Eng. 16(5), 556-561 (1999)) disclose a batch lactic acid esterification process using a strongly acidic cation exchange resin as a step for the recovery of high purity lactic acid. A low concentration of lactic acid solution of 20 weight percent or less is used, since it was believed that overall lactic acid yield decreases with higher feed concentrations of lactic acid. The esterification reaction was carried out in a reboiler and the methyl lactate, water and unreacted methanol were distilled upward through the distillation column. Methanol was recycled to the reboiler, while water and methyl lactate were recovered from the bottom of the reboiler. Seo et al. disclose a weight fraction of methyl lactate in the bottom product of the esterification column of approximately thirty percent or less. This reference discloses only batch distillation.
Choi et al. (J. Chem. Eng. Of Japan 32(2), 184-189 (1999)) disclose a batch lactic acid esterification process using a strongly acidic cation exchange resin as a step for the recovery of high purity lactic acid. A low concentration of lactic acid solution of 20 weight percent or less was used, since overall lactic acid yield decreased with higher feed concentrations of lactic acid. The esterification reaction was carried out in a feed vessel and the methyl lactate, water and unreacted methanol were distilled upward through the distillation column. Methanol was recycled to the feed vessel, while water and methyl lactate were removed to a reboiler where hydrolysis occurs after being condensed.
A more recent approach is the use of membranes (Benedict, D. J., et al., Ind. Eng. Chem. Res. 42 (2003); Feng, X., et al., Chem. Eng. Sci. 51(20), 4673-4679 (1996); Jafar, J. J., et al., J. Membrane Science 199, 117-123 (2002); Walsh, K., Chemical Week 161(40), 23 (1999); and Datta, R., et al., U.S. Pat. No. 5,723,639 (1998)); for example, there is a process that has been developed at Argonne National Laboratory in which water produced in esterification is removed through a membrane. However, membranes are susceptible to fouling. U.S. Pat. No. 5,723,639 to Datta et al., for example, discloses a process of esterification of fermentation derived organic acids such as lactic acid using pervaporation. The methods described by Datta et al., Benedict et al., Feng et al., and Jafar et al. each require a pervaporation membrane.
Other alternative methods of purification of esterification products have been described. U.S. Pat. No. 5,405,992 to Funk et al. discloses a process of continuous esterification with concurrent separation of the products upon a solid bed used as both an esterification catalyst and as an adsorbent for at least one of the esterification products.
The applications of reactive distillation have been focused on esters of acetic acid (Steinigeweg, S., et al., Ind. Eng. Chem. Res. 41:5483-5490 (2002); Popken, T., et al., Ind. Eng. Chem. Res. 40:1566-1574 (2001); Hanika, J., et al., Chem. Eng. Sci. 54:5205-5209 (1999); and Agreda, V. H., et al., U.S. Pat. No. 4,435,595 (1984)). Steinigeweg et al. disclose a reactive distillation process for the production of n-butyl acetate. Steinigeweg et al. vary several operational conditions and two different feed position setups were studied. Popken et al. disclose a one-feed and a two-feed reactive distillation process for the production of methyl acetate. Popken et al. breaks the binary azeotrope of methyl acetate-water by the extraction of the water by acetic acid feed. A pure, dry acetic acid feed is required to achieve a high purity methyl acetate in the distillate. Hanika et al. disclose a catalytic distillation process for the production of butylacetate. U.S. Pat. No. 4,435,595 to Agreda et al. discloses a reactive distillation process for producing high purity methyl acetate which achieves high reactant conversion. Neither Steinigeweg et al., Popken et al. Hanika et al. or Agreda et al. disclose a continuous process of producing high purity lactic acid or acid esters other than acetic acid ester within a single reaction column.
There has also recently been a study reported for the formation of fatty acid esters using reactive distillation (Omota, F., et al., Chem. Eng. Sci. 58 3159-3174 (2003); and Omota, F., et al., Chem. Eng. Sci. 58 3175-3185 (2003)). Omota et al. (Parts 1 and 2) describe fatty acid esterification by reactive distillation in the presence of a metal catalyst and in the absence of water. Smejkal, Q., et al., Chem. Eng. Science 56 365-370 (2001) describe the preparation of methyl propyl acetate from acetic acid and 2-methyl propanol. No aqueous feed solutions were used.
U.S. Pat. No. 5,008,046 to Bremus et al. describes a reactive distillation process using a column with plate type column with an acid catalyst under pressure. U.S. Pat. No. 5,536,856 to Harrison et al. describes a similar column wherein resin acid catalyst particles were supported by a tray on the column. The use of columns with trays is quite expensive.
Transesterification has been carried out in distillation columns. U.S. Pat. No. 4,370,491 to Bott et al. discloses a process for the preparation of acetic acid esters by alkali-catalyzed trans-esterification of an acetic acid ester with an alcohol carried out in the middle section of a distillation column wherein the alkaline catalyst is introduced into the upper part of the column, an alcohol is fed into the upper zone, and an ester is fed into the lower zone. Bott et al. does not disclose a continuous process of lactic acid ester trans-esterification within a single reaction column.
While the related art teaches organic acid esterification processes, there still exists a need for improved continuous processes for lower acid (C3 to C8) esterification and trans-esterification of the resulting esters.